Silicon carbide (SiC) has a dielectric breakdown field strength about ten times as large as that of silicon (Si), and in addition to that, is a semiconductor having excellent physical properties also in thermal conductivity, electron mobility, band gap and the like. Therefore, it is expected as a semiconductor material which can achieve dramatic improvement of performance compared with a conventional silicon-based power semiconductor device.
In recent years, development of a high performance silicon carbide semiconductor device has been advanced: for example, a 4H-silicon carbide single crystal substrate and a 6H-silicon carbide single crystal substrate having a diameter up to 3 inches have become commercially available, and there have been successively reported various semiconductor switching devices which substantially exceed the performance limits of Si.
A semiconductor device is classified roughly into a unipolar semiconductor device, in which only an electron or a hole acts on electric conduction during current passage, or a bipolar semiconductor device, in which both an electron and a hole act on electric conduction. The unipolar semiconductor devices include a Schottky barrier diode (SBD), a junction field effect transistor (J-FET), and a metal/oxide/semiconductor field effect transistor (MOS-FET). The bipolar semiconductor devices include a pn diode, a bipolar junction transistor (BJT), a thyristor, a gate turn-off thyristor (GTO thyristor), and an insulated gate bipolar transistor (IGBT).
When a power semiconductor device is produced using a silicon carbide single crystal, since impurities are difficult to be deeply diffused because of the extremely small diffusion coefficient of the silicon carbide single crystal, a single crystal film having the same crystal type as the substrate and having a predetermined film thickness and doping concentration is frequently epitaxially grown on the silicon carbide bulk single crystal substrate (Patent Document 1). Specifically, a silicon carbide single crystal substrate is used in which an epitaxial single crystal film is grown by a CVD method on a surface of a substrate that is a slice of a bulk single crystal obtained by a sublimation method or chemical vapor deposition (CVD) method.
There are various crystal polymorphs (polytypes) of silicon carbide single crystal. In the development of the power semiconductor device, there is mainly used a 4H-silicon carbide which has a high dielectric breakdown field strength, high mobility and a relatively small anisotropy. The crystal faces subjected to epitaxial growth include a (0001) Si face, (000-1) C face, (11-20) face, (01-10) face, and (03-38) face. When the single crystal film is grown epitaxially from the (0001) Si face and (000-1) C face, a crystal face in which these faces are inclined by a few degrees of angle towards the direction of [11-20] or [01-10] is frequently used in order to grow the crystal homo-epitaxially with the step flow growth technology.    [Patent Document 1]    International Publication No. WO03/038876 Pamphlet    [Non-patent Document 1]    Journal of Applied Physics, Vol. 95, No. 3, pp. 1485-1488, 2004.    [Non-patent Document 2]    Journal of Applied Physics, Vol. 92, No. 8, pp. 4699-4704, 2002.    [Non-patent Document 3]    Journal of Crystal Growth, Vol. 262, pp. 130-138, 2004.
Although the power semiconductor device using silicon carbide has various excellent properties as mentioned above, there have been the following problems. Various crystal defects are generated in the production process within the silicon carbide single crystal in the bipolar silicon carbide semiconductor device. Specifically, in the first place, various crystal defects are generated in the step of growing silicon carbide bulk single crystal by using a modified Rayleigh method or a CVD method. In a bipolar silicon carbide semiconductor device produced by using a wafer which is cut out from a silicon carbide bulk single crystal containing these various crystal defects, the crystal defects present within the wafer cause the deterioration of device properties.
In the second place, various crystal defects are generated in a silicon carbide epitaxial film in the step of growing it from a surface of a silicon carbide bulk single crystal substrate by a CVD method. The crystal defects generated here include various kinds such as a line defect, a point defect, and a ring defect.
FIGS. 1 (a) and (b) are cross-sectional views showing the vicinity of the interface between a silicon carbide single crystal substrate and a silicon carbide epitaxial film formed on the surface of the substrate with the step flow growth technology. In FIG. 1 (a), 5 is a crystal face ((0001) Si face) and θ is an off-angle. As shown in FIG. 1 (a), there exist various crystal defects including a line defect 6, a point defect 7, and a ring defect 8 in epitaxial films (n type epitaxial film 2a and p type epitaxial film (or a p type implanted layer) 2b) formed on a silicon carbide single crystal substrate 1. The line defects 6 include, for example, a basal plane dislocation extending in parallel with the (0001) Si face. In addition, as shown in FIG. 1 (b), there exist many ring defects 8 in the vicinity of the surface of the silicon carbide epitaxial film 2.
In a bipolar device such as a pn diode, an n type epitaxial film, and a vicinity of the interface between the n type epitaxial film and a p type epitaxial film or a vicinity of the interface between the n type epitaxial film and a p type implanted layer are regions in which an electron and a hole recombine with each other during current passage. However, defects which are Shockley partial dislocations (also referred to as a Shockley imperfect partial dislocations) having a Burgers vector of [01-10] cause a stacking fault by being affected by the recombination energy of an electron and a hole generated during current passage (the above-mentioned Non-patent Documents 1 to 4). As shown in FIG. 4, the stacking fault appears as a planar defect having a shape of a triangle, rhombus and the like.
The region of the stacking fault is considered to act as a high resistance area during current passage. As a result, a forward voltage of a bipolar semiconductor device is increased as the area of the stacking fault expands.
In the third place, after a silicon carbide epitaxial film is formed on a surface of a silicon carbide bulk single crystal substrate, a bipolar silicon carbide semiconductor device is produced through various steps such as, for example, formation of a mesa structure, ion implantation, formation of an oxide film, and formation of an electrode. However, the above-mentioned crystal defects, that is, the line defect, the point defect, the ring defect and the like are generated even in these device processing steps onto the silicon carbide single crystal. For example, since the diffusion coefficient for the impurity atoms is small in the silicon carbide bulk single crystal and therefore the doping of impurities by a thermal diffusion method is difficult to apply to the silicon carbide bulk single crystal substrate, a nitrogen ion or an aluminum ion may be introduced into the silicon carbide epitaxial film by ion implantation. In addition, even in the forming of JTE in a pn diode, ions are implanted into the silicon carbide epitaxial film. In implanting these ions, impurity ions implanted into the crystal collide with the crystal, thereby breaking the crystal structure of the silicon carbide single crystal and damaging the silicon carbide single crystal. As a result, the above-mentioned crystal defect is expected to be generated.
As mentioned above, various crystal defects are generated within a silicon carbide single crystal in the step of forming a silicon carbide single crystal substrate, in the step of forming a silicon carbide epitaxial film and in the subsequent step of device processing a silicon carbide crystal. The crystal defects cause the deterioration of properties of the bipolar silicon carbide semiconductor device produced. Especially, the crystal defects present in the silicon carbide epitaxial film are transformed into planes by current passage to cause a stacking fault, and if the area of such fault is expanded the forward voltage is increased. The increase of the forward voltage decreases the reliability of the silicon carbide bipolar semiconductor device and causes the increase of power loss of power control equipment incorporated with the bipolar silicon carbide semiconductor device. For this reason, there has been a need that the defects should be reduced which are the nuclei of a stacking fault which is expanded by current passage.
The present invention is made for solving the above-mentioned problems in the conventional technology and it is an object of the present invention to reduce the defects which are the nuclei of a stacking fault which is expanded by current passage, thereby preventing the increase of a forward voltage of a bipolar silicon carbide semiconductor device.